1. Field of the Invention
The invention relates to a magnetic resonance imaging method.
2. Description of Related Art
A magnetic resonance imaging method is disclosed in the European patent application EP 0 526 983.
In order to generate a magnetic resonance image of an object to be examined, the object is placed in a steady, essentially spatially uniform magnetic field. This steady magnetic field to a certain extent aligns magnetic spins in the object in the direction of the magnetic field. In the known magnetic resonance imaging method a 90xc2x0 RF excitation pulse is applied to excite magnetic spins in a selected part of an object to be examined, such as a patient to be medically examined. The part of the object in which the magnetic spins are excited is selected by way of a temporary selection gradient which is superimposed on the steady magnetic field. A 180xc2x0 RF refocusing pulse is applied in order to generate magnetic resonance spin echo signals. The magnetic resonance spin echo signals are received by a receiver while a temporary read gradient is superimposed on the steady magnetic field. The read gradient is directed in a read direction. The temporary read gradient frequency-encodes the magnetic resonance spin echo signals in the read direction. Phase-encoding of the magnetic resonance spin echo signals in the phase-encoding direction is carried out by way of phase-encoding gradients superimposed on the steady magnetic field. The phase-encoding direction extends at right angles to the read direction. The phase-encoding gradient and the read gradient effect spatial encoding of the magnetic resonance spin echo signals. The magnetic resonance image is reconstructed from these spatially encoded magnetic resonance spin echo signals.
The known method concerns the avoidance of so-called out-of-slice artefacts in the magnetic resonance image. Such out-of-slice artefacts are mainly caused by imperfections of the 180xc2x0 RF refocusing pulse. Such imperfections give rise to a spurious transverse magnetisation which leads to disturbing magnetic resonance signals. According to the known method the phase-encoding gradient is applied before the 180xc2x0 RF refocusing pulse. Consequently, the disturbing magnetic resonance signals due to the imperfections of the 180xc2x0 RF refocusing pulse are not phase-encoded so that in the magnetic resonance image the out-of-slice artefacts are collapsed into a single column or row of the magnetic resonance image. Although the known method is quite effective in avoiding the out-of-phase artefacts, the known method appears to be restricted to the reduction of corruptions of the image that are due to imperfections of the 180xc2x0 RF refocusing pulse.
Furthermore, in the known method a predetermined phase-offset is applied to the 90xc2x0 RF excitation pulse so that the phase of the excited magnetic spins is offset by the predetermined phase-offset. The receiver also changes the phase of the received magnetic resonance spin echo signal by the predetermined phase-offset. The phase of the 180xc2x0 RF refocusing pulse is not offset. Consequently, the receiver offsets the phase of the spurious magnetic resonance signals due to imperfections of the 180xc2x0 RF refocusing pulse by the predetermined phase-offset. The predetermined phase-offset of the magnetic resonance spin echo signals due to the 90xc2x0 RF excitation is cancelled by the predetermined phase-offset of the receiver. Hence, the artefact that is collapsed into a single line is shifted to an edge of the image where it is the least annoying.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant""s invention of the invention subsequently claimed.
An object of the invention is to provide a magnetic resonance imaging method which is substantially insusceptible to disturbances due to spuriously generated magnetisation, such as the so-called xe2x80x98Eemland-artefactxe2x80x99.
This object is achieved by the magnetic resonance imaging method according to the invention wherein
a preparatory excitation of spins is applied,
subsequently a temporary magnetic compensation gradient is applied in a direction which opposes a phase-encoding direction,
subsequently an RF excitation pulse is generated,
a phase-encoding gradient is applied in the phase-encoding direction, and
magnetic resonance signals are received during an acquisition interval.
The temporary magnetic compensation gradient causes a pre-compensating phase-encoding of any spuriously excited spins, i.e. spuriously generated magnetization, in the object under examination. Such spuriously excited spins are, for example, generated in the preparatory excitation. The combined effect of the pre-compensating phase-encoding and the phase-encoding that is applied after the RF-excitation is that the spuriously excited spins have no phase-encoding after the phase-encoding gradient. Hence, these spuriously excited spins cannot give rise to phase-encoded magnetic resonance signals. Consequently, these spuriously excited magnetizations are prevented from causing serious image corruptions such as the xe2x80x98Eemland-artefactxe2x80x99. As the magnetic resonance signals due to such spuriously excited spins are effectively not phase-encoded, these magnetic resonance signals give rise to only small stripes in the magnetic resonance image that is reconstructed from the magnetic resonance signals. The reduction of image corruptions due to spuriously generated magnetization enhances the diagnostic quality of the magnetic resonance image in that small details in the object being imaged are faithfully rendered visible in the magnetic resonance image. According to the invention it is notably avoided that the magnetic resonance image contains spurious details which are not related to the anatomy of the patient being examined. It is to be noted that the magnetic resonance image forms a technical feature which is of assistance to a physician, for example an MRI-radiologist, to reach a diagnosis.
These and other aspects of the invention will be elaborated in more detail on the basis of the following embodiments.
Preferably, a predetermined phase-offset is applied to the RF-excitation pulse and a compensating phase-offset is applied to the receiver so as to shift the small stripes to an edge of the image where the corruption of the image by such small stripes hardly affects the diagnostic quality of the image. As an alternative, the stripe-like artefact may be shifted to the edge of the image during the reconstruction of the image from the magnetic resonance signals. Notably the RF-excitation and the receiver as well as the preparatory excitation involve respective predetermined phases. Thus, there is a definite phase-difference between the magnetic resonance signals due to the RF-excitation and spurious excitations due to the preparatory excitation. The spurious excitations cause a disturbance that according to the invention is not phase-encoded and hence is collapsed into a stripe. Owing to the definite phase-difference in the reconstruction of the image from the magnetic resonance signals the stripe can be shifted to the edge of the image.
The magnetic resonance imaging method of the invention is particularly effective in mitigating artefacts due to the application of so-called REST slabs (Regional Saturation Technique). Such REST slabs are often applied to avoid perturbation due to motion of a portion of the object to be examined. REST involves preparatory excitation and subsequent dephasing of spins in a selected region. Magnetic spins in the selected region are excited by way of a preparatory RF-excitation pulse and a preparatory selection gradient. Such a selected region is notably a region of the object where motion is expected to occur during magnetic resonance imaging. Usually this portion is indicated as a REST slab. Subsequently, the excited magnetic spins are dephased, notably by a dephasing gradient field. Because the magnetic spins in the REST slab are dephased to a large extent, the magnetic spins in the REST slab hardly emit magnetic resonance signals. However, it has been found that in some practical situations the dephasing of the magnetic spins in the portion selected by the preparatory excitation and the preparatory selection gradient, i.e. the magnetic spins in the REST slab, is not complete so that inadvertently magnetic resonance signals are generated from the REST slab. For example, spurious residual in-phase magnetisation may be generated in the REST slab owing to imperfections in the spatial uniformity of the static magnetic field. If no steps are taken, such residual in-phase magnetisation in the REST slab will give rise to the so-called xe2x80x98Eemlandxe2x80x99-artefact.
Another example of a preparatory excitation which, if no steps are taken, may cause artefacts in the magnetic resonance image is the so-called SPIR pre-pulse (SPectral Inversion Recovery) in which, for example, spins and fat or water are selectively excited. Furthermore, the preparatory excitation may be an MTC (Magnetization Transfer Contrast) pre-pulse.
Preferably, the time-integral of the gradient fields in the phase-encoding direction over the interval from the end of the preparatory excitation until a point in time within the acquisition interval equals a predetermined net gradient. The predetermined net gradient has the same value for all phase-encoding steps; in other words, in the (reciprocal) k-space the predetermined net gradient is independent of ky, where ky is the component of the wave vector corresponding to the phase-encoding direction. For example, the predetermined net gradient equals zero. The sequence starting with the preparatory sequence until the acquisition of the magnetic resonance signals is usually repeated. In practice such a repetition is carried out very often in order to acquire enough magnetic resonance signals for the reconstruction of the magnetic resonance image with a high diagnostic quality. For example, the sequence is repeated 128 or 256 times and the magnetic resonance signal is sampled for 128 or 256 (frequency) samples in the read direction. Owing to said time-integral of the gradients in the phase-encoding direction, being the same in all repetitions of the sequence, the spuriously generated magnetisation remains not phase-encoded over the repetitions of the sequence. Hence, according to the invention the artefacts which are due to spuriously generated magnetizations and are reduced to stripes can be moved to the edge of the image by the same phase-shift for all repetitions of the sequence. Reduction of the artefacts into small stripes at the edge of the magnetic resonance image is thus simply achieved.
The invention also relates to a magnetic resonance imaging system. The magnetic resonance imaging system according to the invention is arranged to apply a preparatory excitation of spins, to apply subsequently a temporary magnetic compensation gradient in a direction which opposes a phase-encoding direction, to generate subsequently an RF excitation pulse, to apply a phase-encoding gradient, and to receive magnetic resonance signals during an acquisition interval. This system is capable of performing the magnetic resonance imaging method of the invention which achieves a substantial reduction of artefacts due to spuriously generated magnetization, e.g. due to preparatory excitations.
It is to be noted that the functions of the magnetic resonance imaging system are preferably carried out by a suitably programmed computer.
The invention also relates to a computer program for performing the magnetic resonance imaging method of the invention. The computer program according to the invention includes instructions to apply a preparatory excitation of spins, to apply subsequently a temporary magnetic compensation gradient in a direction which opposes a phase-encoding direction, to generate subsequently an RF excitation pulse, to apply a phase-encoding gradient, and to receive magnetic resonance signals during an acquisition interval. Advantageously, the magnetic resonance imaging system includes a computer into which the computer program according to the invention is loaded.